Catalytic Reduction of p-Nitrophenol on MnO2/Zeolite -13X Prepared with Lawsonia inermis Extract as a Stabilizing and Capping Agent

p-nitrophenol (pNP) is a highly toxic organic compound and is considered carcinogenic and mutagenic. It is a very stable compound with high resistance to chemical or biological degradation. As a result, the elimination of this pollutant has been very challenging for many researchers. Catalytic reduction is one of the most promising techniques, if a suitable catalyst is developed. Thus, this work aims to prepare an eco-friendly catalyst via a simple and low-cost route and apply it for the conversion of the toxic p-nitrophenol (pNP) into a non-toxic p-aminophenol (pAP) that is widely used in industry. Manganese oxide was prepared in an environmentally friendly manner with the aid of Lawsonia inermis (henna) extract as a stabilizing and capping agent and loaded on the surface of 13X molecular sieve zeolite. The UV-Vis spectrum, EDS, and XRD patterns confirmed the formation of the pure MnO2 loaded on the zeolite crystalline network. The TGA analysis showed that the samples prepared by loading MnO2 on zeolite (Mn2Z, Mn3Z, and Mn4Z) lost more mass than pure MnO2 (Mn) or zeolite (Z), which is mainly moisture adsorbed on the surface. This indicates a better dispersion of MnO2 on the surface of zeolite compared to pure MnO2, and thus a higher number of active adsorption sites. SEM images and EDS confirmed the dispersion of the MnO2 on the surface of the zeolite. Results showed a very fast reduction rate, following the order Mn2Z > Mn3Z > Mn4Z > Mn > Z. With sample Mn2Z, 96% reduction of pNP was achieved in 9 min and 100% in 30 min. For Mn3Z, Mn4Z, and Mn, 98% reduction was achieved in 20 min and 100% in 30 min. Zeolite was the slowest, with only a 40% reduction in 30 min. Increasing the amount of zeolite in the synthesis mixture resulted in lower reduction efficiency. The kinetic study indicated that the reduction of p-nitrophenol on the surface of the prepared nanocomposite follows the pseudo-first-order model. The results show that the proposed nanocomposite is very effective and very promising to be commercially applied in water treatment, due to its low cost, simple synthesis procedure, and reusability.


Introduction
p-nitrophenol (pNP) is a highly toxic organic compound that is discharged to the water system from the pharmaceutical, photographic, fungicides, medicines, hair dyeing agents, leather, and corrosion inhibitors industries [1]. It causes many skin diseases and is considered carcinogenic and mutagenic [2]. Owing to the presence of the nitro group, pNP is a very stable compound with high resistance to chemical or biological oxidation [3]. Accordingly, many techniques such as adsorption [4], photocatalytic degradation [5], ozonation [6], membrane [7], microbial treatment [5], and electrocoagulation [8] have been applied for the removal of pNP from water. Among these processes, the conversion of the pNP into MnO 2 nanoparticles have been applied widely in catalysis, adsorption, oxidation reactions, dry cells, pigment, ceramics, electrodes, and batteries [43]. However, MnO 2 has a major drawback related to its irregular pore structure that decreases the available surface area and, accordingly, the catalytic efficiency [40]. Furthermore, fine particles formed at relatively low temperatures tend to aggregate at high temperatures, leading to coarse crystalline materials with a particle size in the range of 2.5-10 µm. Similarly, the surface area decreases with an increase in the particle size distribution [54]. A promising solution for this shortcoming is to combine MnO 2 with suitable support. In this regard, so far, a few studies have reported the synthesis of MnO 2 /zeolite composite. Thus, in this work, an effective and eco-friendly catalyst will be prepared by combining 13X zeolite molecular sieve as support with MnO 2 nanoparticles prepared with the aid of L. inermis extract as a stabilizing and capping agent. The attractive characteristics of the proposed catalyst include the very simple and low-cost synthesis procedure, which eliminates the need for harsh chemicals such as NABH 4 or critical conditions such as high temperature, the flow of inert gas, or multiple steps.

Preparing Lawsonia inermis (Henna) Extract
To prepare the Henna extract (HE), 10 g of the plant root powder was mixed with 100 mL of distilled water. The mixture was boiled for 10 min then left overnight. After that, the mixture was filtered with Whatman filter paper and the filtrate was kept in the fridge until it was used.

Preparing the MnO 2 Nanoparticles (M)
Henna extract (25mL) was mixed with 100 mL of 0.1 M MnCl 2 ·4H 2 O solution. The pH of the mixture was adjusted to 11 using 1 M NaOH and an Orion 2-Star pH meter. The mixture was then stirred at room temperature and 70 rpm for 24 h. After that, the mixture was filtered and the solid was washed with distilled water and then ethanol. Finally, it was dried in a convection oven at 100 • C for 3 h, then collected and kept in a sealed container until it was used. Figure 1 shows the steps followed in the synthesis of MnO 2 /Zeolite. Specifically, 100 mL of 0.1 M MnCl 2 ·4H 2 O solution was mixed with 25 mL of the henna extract. A certain mass of zeolite (13X molecular sieves), according to Table 1, was added to the mixture and it was stirred for 30 min. The pH of the mixture was adjusted to 11 using 1 M NaOH and an Orion 2-Star pH meter. The mixture was then stirred at room temperature and 70 rpm for 24 h. After that, the mixture was filtered and the solid was washed with distilled water and then ethanol. Finally, it was dried in a convection oven at 100 • C for 3 h, then collected and kept in a sealed container until it was used.

Characterization
To confirm the formation of MnO2, all the nanocomposite samples were analyzed by X-ray diffraction (XRD) using XRD-7000 with a Cu-detector (Shimadzu, Tokyo, Japan) over the 2Ɵ range of 10-80°. Furthermore, they were dispersed in distilled water and the UV-Vis spectra were recorded for the wavelength range of 200-1100 nm using a UV-Vis spectrophotometer (Shimadzu, Tokyo, Japan). To test the thermal stability of the prepared samples, thermal gravimetric analysis (TGA) was conducted under nitrogen with a temperature ramp of 2 °/min from 20 °C to 500 °C using a TGA-51 Shimadzu Thermogravimetric Analyzer (Tokyo, Japan). Dynamic light scattering (DLS) analysis was performed for the MnO2 nanoparticles to calculate the particle size distribution. This analysis was performed with a Thermo Scientific, Quattro S, USA. Scanning electron microgram (SEM) imaging was performed for all samples using a Thermo Scientific, Quattro S, USA. FTIR analysis was performed with a Thermo Scientific, Quattro S, USA for the wavenumber range of 4000-4000 cm −1 . Nitrogen adsorption was measured for all samples after degassing the samples at 70 K, and the BET technique was performed to estimate the surface area. This analysis was performed with a NOVA 4200e (Quantachrome Instruments, B Beach, FL, USA).

Catalytic Reduction of p-Nitrophenol (pNP)
To study the catalytic activity of the prepared samples, 3 mL of 20 mgL −1 pNP solution was mixed with 1 mg of NaBH4 and 10 mg of Z, Mn2Z, Mn3Z, Mn4Z, or Mn was

Characterization
To confirm the formation of MnO 2 , all the nanocomposite samples were analyzed by X-ray diffraction (XRD) using XRD-7000 with a Cu-detector (Shimadzu, Tokyo, Japan) over the 2

Characterization
To confirm the formation of MnO2, all the nanocomposite samples were analyzed by X-ray diffraction (XRD) using XRD-7000 with a Cu-detector (Shimadzu, Tokyo, Japan) over the 2Ɵ range of 10-80°. Furthermore, they were dispersed in distilled water and the UV-Vis spectra were recorded for the wavelength range of 200-1100 nm using a UV-Vis spectrophotometer (Shimadzu, Tokyo, Japan). To test the thermal stability of the prepared samples, thermal gravimetric analysis (TGA) was conducted under nitrogen with a temperature ramp of 2 °/min from 20 °C to 500 °C using a TGA-51 Shimadzu Thermogravimetric Analyzer (Tokyo, Japan). Dynamic light scattering (DLS) analysis was performed for the MnO2 nanoparticles to calculate the particle size distribution. This analysis was performed with a Thermo Scientific, Quattro S, USA. Scanning electron microgram (SEM) imaging was performed for all samples using a Thermo Scientific, Quattro S, USA. FTIR analysis was performed with a Thermo Scientific, Quattro S, USA for the wavenumber range of 4000-4000 cm −1 . Nitrogen adsorption was measured for all samples after degassing the samples at 70 K, and the BET technique was performed to estimate the surface area. This analysis was performed with a NOVA 4200e (Quantachrome Instruments, B Beach, FL, USA).

Catalytic Reduction of p-Nitrophenol (pNP)
To study the catalytic activity of the prepared samples, 3 mL of 20 mgL −1 pNP solution was mixed with 1 mg of NaBH4 and 10 mg of Z, Mn2Z, Mn3Z, Mn4Z, or Mn was added to the solution; the concentration of pNP was monitored as a function of time by measuring the UV absorbance at 400 nm every 3 min for 30 min, using a UV-Vis spectrophotometer (Shimadzu, Tokyo, Japan). The removal percentage of pNP was calculated with Equation (1): range of 10-80 • . Furthermore, they were dispersed in distilled water and the UV-Vis spectra were recorded for the wavelength range of 200-1100 nm using a UV-Vis spectrophotometer (Shimadzu, Tokyo, Japan). To test the thermal stability of the prepared samples, thermal gravimetric analysis (TGA) was conducted under nitrogen with a temperature ramp of 2 • /min from 20 • C to 500 • C using a TGA-51 Shimadzu Thermogravimetric Analyzer (Tokyo, Japan). Dynamic light scattering (DLS) analysis was performed for the MnO 2 nanoparticles to calculate the particle size distribution. This analysis was performed with a Thermo Scientific, Quattro S, USA. Scanning electron microgram (SEM) imaging was performed for all samples using a Thermo Scientific, Quattro S, USA. FTIR analysis was performed with a Thermo Scientific, Quattro S, USA for the wavenumber range of 4000-4000 cm −1 . Nitrogen adsorption was measured for all samples after degassing the samples at 70 K, and the BET technique was performed to estimate the surface area. This analysis was performed with a NOVA 4200e (Quantachrome Instruments, B Beach, FL, USA).

Catalytic Reduction of p-Nitrophenol (pNP)
To study the catalytic activity of the prepared samples, 3 mL of 20 mgL −1 pNP solution was mixed with 1 mg of NaBH 4 and 10 mg of Z, Mn2Z, Mn3Z, Mn4Z, or Mn was added to the solution; the concentration of pNP was monitored as a function of time by measuring the UV absorbance at 400 nm every 3 min for 30 min, using a UV-Vis spectropho-tometer (Shimadzu, Tokyo, Japan). The removal percentage of pNP was calculated with Equation (1): where A t and A 0 are the absorbance of p-nitrophenolate ion at any time t and at time zero, respectively. The kinetics of the catalytic reduction were tested with pseudo-firstorder kinetics. The linear form of this kinetic model is given in Equation (2). The rate constants (k), were evaluated by plotting Ln (A t ) versus time. The slope value of the straight line represents the rate constant k (min −1 ).
For the recycling experiment, the catalyst was collected by filtration, washed thoroughly with deionized water, and reused in the catalytic cycles. Figure 2 shows the FTIR spectra for Z, Mn2Z, Mn3Z, Mn4Z, Mn, and the plant extract (HE). All the patterns have a broad peak in the wavenumber range of~3200-3600 cm −1 , which is assigned to stretching vibrations of the O-H in hydrogen-bonded internal silanol groups and O-H stretching of water attached to the surface [41] The peak at~1620 cm −1 is related to vibration bending of coordinated (-OH) groups attached to the zeolite surface. The peak at~450 cm −1 in Mn2Z, Mn3Z, Mn4Z, and Mn is assigned to the Mn-O bond [14,42], while the peak at~450 cm −1 in Z is related to the bending vibrations of Si-O and Al-O in zeolites [40]. The peak at~740 cm −1 in Mn2Z, Mn3Z, Mn4Z, and Z is assigned to the stretching vibration of Al-O [55], while the peak at~960 cm −1 in the same samples is related to Si-O stretching vibrations, and the asymmetric stretching of Si-O-Al tetrahedral [55]. In the HE pattern, the peak at~1437 cm −1 is assigned to the C-O bond and that at~1668-1725 cm −1 is related to the stretching vibration of the C=O cm −1 group of the effective compounds in the extract.

FTIR Analysis
where At and A0 are the absorbance of p-nitrophenolate ion at any time t and at time zero, respectively. The kinetics of the catalytic reduction were tested with pseudo-first-order kinetics. The linear form of this kinetic model is given in Equation (2). The rate constants (k), were evaluated by plotting Ln (At) versus time. The slope value of the straight line represents the rate constant k (min −1 ).
For the recycling experiment, the catalyst was collected by filtration, washed thoroughly with deionized water, and reused in the catalytic cycles. Figure 2 shows the FTIR spectra for Z, Mn2Z, Mn3Z, Mn4Z, Mn, and the plant extract (HE). All the patterns have a broad peak in the wavenumber range of ~3200-3600 cm −1 , which is assigned to stretching vibrations of the O-H in hydrogen-bonded internal silanol groups and O-H stretching of water attached to the surface [41] The peak at ~1620 cm −1 is related to vibration bending of coordinated (-OH) groups attached to the zeolite surface. The peak at ~450 cm −1 in Mn2Z, Mn3Z, Mn4Z, and Mn is assigned to the Mn-O bond [14,42], while the peak at ~450 cm −1 in Z is related to the bending vibrations of Si-O and Al-O in zeolites [40]. The peak at ~740 cm −1 in Mn2Z, Mn3Z, Mn4Z, and Z is assigned to the stretching vibration of Al-O [55], while the peak at ~960 cm −1 in the same samples is related to Si-O stretching vibrations, and the asymmetric stretching of Si-O-Al tetrahedral [55]. In the HE pattern, the peak at ~1437 cm −1 is assigned to the C-O bond and that at ~1668-1725 cm −1 is related to the stretching vibration of the C=O cm −1 group of the effective compounds in the extract.  Figure 3 shows the UV-Vis spectra of Mn2Z, Mn3Z, Mn4Z, and Mn for the wavelength range of 200-1100 nm. The presence of the absorbance band in the range of 240-300 nm confirms the successful production of MnO nanoparticles. The shift of the absorption bands shows that different morphologies and sizes are presented [56].  Figure 3 shows the UV-Vis spectra of Mn2Z, Mn3Z, Mn4Z, and length range of 200-1100 nm. The presence of the absorbance band i 300 nm confirms the successful production of MnO nanoparticles. The tion bands shows that different morphologies and sizes are presented

TGA Analysis
The TGA profile of the five samples is shown in Figure 4 for the of 20-500 °C. Pure MnO2 has the highest stability, with a mass loss of

TGA Analysis
The TGA profile of the five samples is shown in Figure 4 for the temperature range of 20-500 • C. Pure MnO 2 has the highest stability, with a mass loss of only 10%, which is mainly related to the water adsorbed on the surface. Bare zeolite lost 22%, mainly in the temperature range of 150-250 • C, which is also attributed to the water adsorbed on the surface. The three samples prepared by loading MnO 2 on the surface of zeolite showed a higher mass loss (24-26%) within the same temperature range, indicating that the nanocomposite samples have higher adsorption capacity toward moisture. This indicates a better dispersion of MnO 2 on the surface of zeolite compared to pure MnO 2 , and thus a higher number of active adsorption sites.

XRD Analysis
The phase purity and crystallinity of the prepared samples were investigated by X-ray diffraction technique (XRD), and the results are shown in Figure 5. The XRD patterns of Mn, Mn2Z, Mn3Z, and Mn4Z show peaks at 2θ values of 19.2, 28.6, 40.8, 50.4, and 66.6, which are attributed to (200), (310), (301), (411), and (002) crystal planes, respectively [57]. The presence of these diffraction peaks confirms the presence of the α-MnO 2 crystal phase, according to the standard data (JCPDS card number 44-0141) [58]. Furthermore, extra peaks appear in the pattern of Mn2Z, Mn3Z, and Mn4Z at 2θ of 11.73, 15.2, 20.1, 23.3, 26.7, 31, and 33.8, corresponding to the planes of (311), (331), (440), (533), (642), (662), and (322), respectively, of the zeolite-13X substrate, which matches well with the pattern shown in sample Z related to pure zeolite [42,57]. These results confirmed the successful impregnation of MnO 2 on the surface of zeolite-13X. mainly related to the water adsorbed on the surface. Bare zeolite lost 22%, mainl temperature range of 150-250 °C, which is also attributed to the water adsorbed surface. The three samples prepared by loading MnO2 on the surface of zeolite sh higher mass loss (24-26%) within the same temperature range, indicating that th composite samples have higher adsorption capacity toward moisture. This indicat ter dispersion of MnO2 on the surface of zeolite compared to pure MnO2, and thus a number of active adsorption sites.

XRD Analysis
The phase purity and crystallinity of the prepared samples were investigate ray diffraction technique (XRD), and the results are shown in Figure 5.

Characterization
To confirm the formation of MnO2, all th X-ray diffraction (XRD) using XRD-7000 wit over the 2Ɵ range of 10-80°. Furthermore, the UV-Vis spectra were recorded for the wavele spectrophotometer (Shimadzu, Tokyo, Japan) samples, thermal gravimetric analysis (TGA) perature ramp of 2 °/min from 20 °C to 500 ° metric Analyzer (Tokyo, Japan). Dynamic lig for the MnO2 nanoparticles to calculate the p performed with a Thermo Scientific, Quattro S imaging was performed for all samples using analysis was performed with a Thermo Scie range of 4000-4000 cm −1 . Nitrogen adsorption sing the samples at 70 K, and the BET techni range 10-80 • . Spectra were shifted vertically for better visibility.

SEM and EDS Analysis
The scanning electron micrograms of Mn, Mn2Z, Mn3Z, Mn4Z, and Z were taken at 5000× and 20,000× magnifications, as shown in Figure 6. The SEM data of bare zeolite show the typical crystalline structure, with smooth edges and crystallite sizes in the range Nanomaterials 2023, 13, 785 8 of 20 of~5-10 µm. [59] After depositing the MnO 2 on the surface, the sharp edges are less clear for all samples. The images also indicate a good distribution of MnO 2 on the surface of the zeolite-13X support and a low degree of MnO 2 particle aggregation, with an average particle size of~0.5 µm. Table 1 shows that the specific surface area, specific pore volume, and average pore diameter of pure zeolite, respectively, are 406.75 m 2 g −1 , 0.722 cm 3 g −1 , and 1.89 nm, indicating a microporous structure [60], while for the pure MnO 2 , the values are, respectively, 4.45 m 2 g −1 , 0.0092 cm 3 g −1 , and 3.17 nm. The EDS shown in Figure 7 confirms the dispersion of MnO 2 on the surface of the zeolite. It is shown that the content of Mn decreased by increasing the ratio of zeolite in the sample, with Mn content of 56.3%, 12.8%, 10.1%, 9.9%, and 0.0% for Mn, Mn2Z, Mn3Z, Mn4Z, and Z, respectively.    The nitrogen adsorption/desorption isotherms are shown in Figure 8a. According to the IUPAC classification, the pure zeolite exhibits type II adsorption isotherm with a very slow increase in the adsorbed volume until a P/P0 of 0.8 is reached [60]. At this point, a sudden increase occurred due to the mesoporous structure of the zeolite sample. Furthermore, the appearance of the adsorption/desorption hysteresis loop is due to capillary condensation within the mesoporous structure. The isotherms for the samples Mn2Z, Mn3Z, and Mn4Z exhibit the same pattern, with a drastic decrease in the volume adsorbed, as  The nitrogen adsorption/desorption isotherms are shown in Figure 8a. According to the IUPAC classification, the pure zeolite exhibits type II adsorption isotherm with a very slow increase in the adsorbed volume until a P/P 0 of 0.8 is reached [60]. At this point, a sudden increase occurred due to the mesoporous structure of the zeolite sample. Furthermore, the appearance of the adsorption/desorption hysteresis loop is due to capillary condensation within the mesoporous structure. The isotherms for the samples Mn2Z, Mn3Z, and Mn4Z exhibit the same pattern, with a drastic decrease in the volume adsorbed, as shown in Table 1. This is mainly related to the blockage of the pores of zeolite by the accumulation of MnO 2 particles. The isotherm of pure MnO 2 displays a non-porous nature, with a very low pore volume, as shown in Table 1. Thus, the combination of MnO 2 with the zeolite resulted in better dispersion of the nanoparticles, leading to a better catalytic activity, as will be discussed next. Combining MnO 2 with the zeolite resulted in a drastic decrease in the surface area, pore volume, and average pore diameter for Mn2Z, Mn3Z, and Mn4Z. This is mainly due to the blocking of the pores of zeolite (1.89 nm) with the MnO 2 particles, which have an average particle size distribution of around 600 nm, as shown in Figure 8b. shown in Table 1. This is mainly related to the blockage of the pores of zeolite by the accumulation of MnO2 particles. The isotherm of pure MnO2 displays a non-porous nature, with a very low pore volume, as shown in Table 1. Thus, the combination of MnO2 with the zeolite resulted in better dispersion of the nanoparticles, leading to a better catalytic activity, as will be discussed next. Combining MnO2 with the zeolite resulted in a drastic decrease in the surface area, pore volume, and average pore diameter for Mn2Z, Mn3Z, and Mn4Z. This is mainly due to the blocking of the pores of zeolite (1.89 nm) with the MnO2 particles, which have an average particle size distribution of around 600 nm, as shown in Figure 8b.

Catalytic Reduction of p-Nitrophenol (pNP)
The catalytic performance of the prepared nanocomposites was investigated using the reduction of p-nitrophenol into p-aminophenol according to the proposed mechanism shown in Figure 9. The characteristic absorption peak of pNP, which has a light yellow color as shown in Figure 10a, is located around 317 nm (Figure 11a). When NaBH4 is added to pNP, without a catalyst, the solution color changed to dark yellow immediately ( Figure  10b). Furthermore, the peak position shifted from 317 to 400 nm (Figure 11a). These changes are related to the formation of 4-nitrophenolate anions, according to Equation (3) [5,61]. When the catalyst was added to the solution in the presence of NaBH4, the peak at 400 nm started to decrease, while a new peak around 300 nm started to appear and increase with time, due to the formation of pAP (Figure 11d-h). [2,5] Furthermore, the color of the solution started to disappear and it became colorless at the end of the reaction, as shown in Figure 10d-h, with the very large amount of gas bubbles confirming the reaction was taking place.
Two blank tests were performed to confirm the necessity of both catalyst and NaBH4. In the absence of NaBH4, adding the catalyst to the p-nitrophenol solution did not result in any reduction after 24 h (Figures 10k, 8i and 10b). This is mainly because the formation of p-nitrophenolate requires high pressure of hydrogen that is achieved by adding NaBH4 [5]. Furthermore, adding NaBH4 without the nanocomposites did not initiate the reduction into p-aminophenol, as observed from the solution color, which did not change after 24 h, as shown in Figure 10i,j. Furthermore, in the presence of nanocomposites without adding NaBH4 to the reaction mixture, there was no reduction or change in the absorbance

Catalytic Reduction of p-Nitrophenol (pNP)
The catalytic performance of the prepared nanocomposites was investigated using the reduction of p-nitrophenol into p-aminophenol according to the proposed mechanism shown in Figure 9. The characteristic absorption peak of pNP, which has a light yellow color as shown in Figure 10a, is located around 317 nm (Figure 11a). When NaBH 4 is added to pNP, without a catalyst, the solution color changed to dark yellow immediately (Figure 10b). Furthermore, the peak position shifted from 317 to 400 nm (Figure 11a). These changes are related to the formation of 4-nitrophenolate anions, according to Equation (3) [5,61]. When the catalyst was added to the solution in the presence of NaBH 4 , the peak at 400 nm started to decrease, while a new peak around 300 nm started to appear and increase with time, due to the formation of pAP (Figure 11d-h). [2,5] Furthermore, the color of the solution started to disappear and it became colorless at the end of the reaction, as shown in Figure 10d-h, with the very large amount of gas bubbles confirming the reaction was taking place.
plete the reduction in this study is comparable to most of the previously reported results. However, the catalyst reported in this work can be prepared from low-cost precursors and via very simple procedures, without the need for complicated conditions such as high temperature, multiple steps, a flow of inert gas, or the use of harmful chemicals such as NaBH4 or KBH4, as shown in Table 3. Furthermore, the developed catalyst showed very good reusability, by maintaining almost 95% of the original activity after four successive cycles, as shown in Figure 13.    Two blank tests were performed to confirm the necessity of both catalyst and NaBH 4 . In the absence of NaBH 4 , adding the catalyst to the p-nitrophenol solution did not result in any reduction after 24 h (Figures 10k, 8i and 10b). This is mainly because the formation of p-nitrophenolate requires high pressure of hydrogen that is achieved by adding NaBH 4 [5]. Furthermore, adding NaBH 4 without the nanocomposites did not initiate the reduction into p-aminophenol, as observed from the solution color, which did not change after 24 h, as shown in Figure 10i,j. Furthermore, in the presence of nanocomposites without adding NaBH 4 to the reaction mixture, there was no reduction or change in the absorbance peak ( Figure 11c). Thus, for reduction to occur, both NaBH 4 and nanocomposite are needed. Figure 11 shows the time-dependent UV-Vis absorption spectra for the reduction of pNP with the aid of Z, Mn2Z, Mn3Z, Mn4Z, and Mn. The difference between Z, Mn2Z, Mn3Z, Mn4Z, and Mn is the catalytic activity and, thus, the time required to reach 100% reduction of pNP and the disappearance of the absorption peak at 400 nm. This peak decreased very slowly in sample Z, while Mn2Z showed the fastest rate of reduction. The high efficiency of Mn2Z may be related to the ratio of MnO 2 , which resulted in good loading of MnO 2 , with good distribution on the surface, leading to efficient accessibility of the reactants to the active sites and high catalytic performance. Figure 12a shows the reduction percentage of pNP as a function of time for the five samples. It shows that Mn2Z is very fast, with a 96% reduction achieved in 9 min, compared to 82%, 65%, 61%, and 3% achieved by Mn3Z, Mn4Z, Mn, and Z, respectively. A complete reduction was observed by 30 min for all samples except the bare zeolite, with only a 40% reduction achieved. The performance of the nanocomposites prepared by combining the zeolite as support with the MnO 2 (Mn2Z, Mn3Z, and Mn4Z) showed higher activity than Z and Mn. This is mainly related to improving the dispersion of MnO 2 and thus minimizing the aggregation, resulting in a higher surface area and a higher number of active sites available for the reduction process. A pseudo-first-order kinetic model was used to fit the data, as shown in Figure 12b and Table 2. The results show that all the samples fit well with the pseudo-first-order kinetic model, with an R 2 value of more than 0.90. The k values support the activity order of Mn2Z > Mn3Z > Mn ∼ = Mn4Z >> Z.     Table 3 shows a comparison of the reduction percentage obtained in this work with some other catalysts reported in the literature. As can be seen, the time required to complete the reduction in this study is comparable to most of the previously reported results. However, the catalyst reported in this work can be prepared from low-cost precursors and via very simple procedures, without the need for complicated conditions such as high temperature, multiple steps, a flow of inert gas, or the use of harmful chemicals such as NaBH 4 or KBH 4 , as shown in Table 3. Furthermore, the developed catalyst showed very good reusability, by maintaining almost 95% of the original activity after four successive cycles, as shown in Figure 13. 100% in 10 min. [63] AuCu@Pt nanoalloys 100 mL of 10 nM AuNPs,100 mL of 1 mM 4-NP, 3.5 mL NaBH 4 (100 mM). 100% in 10 min. [15] Complicated synthesis, expensive precursors. and AgNPs (2.5 µL, 6 nmol). 96% in 3 min. [20] Use of NaBH 4 during synthesis.

Conclusions
Zeolite-supported MnO 2 nanocomposite catalysts were synthesized successfully by utilizing plant extract as a stabilizing and capping agent. The synthesis route followed in this work was very simple, eco-friendly, cost-effective, and utilized very mild conditions. The catalytic efficiency of the nanocomposites was tested through the reduction of p-nitrophenol into p-aminophenol. Results indicated that almost 100% reduction was achieved in less than 30 min. Results of this study showed that a zeolite-supported MnO 2 catalyst is very promising for the reduction of organic pollutants. Furthermore, the reusability of the catalyst was tested, and the results showed that 95% of the activity was maintained after four successive cycles, which is very important for commercial application.